The present invention is intended to enable the user-specified appearance tuning of a bitmap image output by a printing system. Using an appearance tuning technique, it is possible to render printed output on a particular image output terminal (IOT) that can be made to emulate the output characteristics of a variety of well-known printers. For example, the present invention allows a user to select a printer emulation so that the resulting output appears as though it had been rendered by a printer of the type emulated (e.g. dark prints for HP LaserJet emulation or light prints for offset printing).
Using morphological-like techniques such as erosion and dilation, the present invention facilitates the identification of specific pixel structures within a bitmap image. Once identified, the structures may be modified or "tuned" so as to appear in the fashion desired once they are rendered by an IOT.
Heretofore, a number of patents and publications have disclosed template-based image filters for image enhancement and resolution conversion, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. Nos. 4,544,264 and 4,625,222 to Bassetti et al., disclose enhancement circuits suitable for use in a laser based electrophotographic printing machine. The enhancements are directed at modifying the digital drive signals used to produce the image, including smoothing digitized edges and broadening fine lines in both the horizontal and vertical directions. Leading and trailing edge signals, in both directions are provided to potentially print each black pixel or line as a series of three pixels, a gray leading pixel, overlapped by a central black pixel, which is in turn overlapped by a gray trailing pixel. A similar process is applied for scan lines as well. The series of signals are recombined to effectively control the voltage and current levels of a laser driver.
U.S. Pat. Nos. 4,847,641 and 5,005,139 to Tung, teach print enhancement circuitry for a laser beam printer. The bit map of a region of the image to be output is compared to a number of patterns or templates. When a match is detected, a section of the bitmap that was matched is replaced with a unique bitmap section designed to compensate for errors. The replacement bitmap section may include predetermined shifting of some dot positions to compensate for the error in the original bitmap section.
U.S. Pat. No. 5,029,108 to Lung teaches an edge enhancement method and apparatus for dot matrix devices wherein a group of gradient mask matrices are applied to a matrix of pixels surrounding a "to be adjusted pixel" so as to determine the existence of an edge and the direction of the brightness change. Once determined, the factors are used to generate a code used to modify the to be adjusted pixel in order to enhance the smoothness of a segment transition.
U.S. Pat. No. 5,408,329 to Mailloux et al., discloses the use of digital darkness control or pixel stretch techniques in electronic imaging devices regardless of differences in development systems (e.g., write-white or write-black). The inventors note that it it is important to match images or have images look alike regardless of the particular system in a given machine that is used in developing the image. Accordingly, the invention provides an electronic adjustment to an original image in order to compensate for various development systems as well as to compensate for quality settings within a given development system. Employed are sets of decoding rules applicable to 2.times.2 or 3.times.3 arrays of image pixels to accomplish darkness adjustment for a given pixel matrix.
U.S. patent application Ser. No. 07/513,415, and the corresponding Japanese laid-open patent publication 4-227584 published Aug. 17, 1992, to Mailloux et el. disclose a method to enable the conversion of binary image data originally generated at a lower resolution into representative binary image data of a higher resolution, wherein the conversion ratio, or magnification factor, is an integer value. Included within the resolution magnification invention are methods for smoothing the interpolated output image and thereby reducing objectionable visual characteristics observable in digitally encoded data using conventional magnification techniques.
A number of patents and publications are summarized in Torrey Pines Research, Behind Hewlett-Packard's Patent on Resolution Enhancement.TM. Technology, (Becky Colgan ed., BIS CAP International, 1990) pp. 1-60, including concepts associated with resolution enhancement.
James C. Stoffel et al. in A Survey of Electronic Techniques for Pictorial Image Reproduction, IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981 discloses image processing algorithms that can be used to transform continuous tone and halftone pictorial image input into spatially encoded representations compatible with binary output processes. A set of image quality and processing complexity metrics are also defined so as to evaluate a number of image processing algorithms with respect to their ability to reproduce continuous tone or halftone pictorial input.
Mathematical Morphology in Image Processing, pp. 43-90 (Edward R. Dougherty ed., Marcel Dekker 1992) describes efficient design strategies for the optimal binary digital morphological filter. A suboptimal design methodology is investigated for binary filters in order to facilitate a computationally manageable design process.
In accordance with the present invention, there is provided a method for selectively tuning the printed output of an electronic printing system capable of producing printed output from a print engine in response to a bitmap image consisting of a plurality of input image signals, comprising the steps of:
storing, in memory, at least a portion of the input image signals forming the bitmap image; PA1 identifying, using a template matching operation applied to the signals stored in the bitmap image memory, pixel values associated with pixel positions to be selectively tuned; PA1 for those pixel positions identified to be selectively tuned, producing a pixel code and transmitting the pixel code to a programmable look-up table to generate a pulse-width position-modulated output signal that is supplied to the print engine; otherwise PA1 passing the pulse-width position-modulated output signal signals directly to the print engine. PA1 a bitmap image memory for storing at least a portion of the plurality of input image signals; PA1 a template matching logic circuit to identify, within the bitmap image memory, image signals associated with pixel positions to be selectively tuned; PA1 a pixel code generator, responsive to said template matching logic circuit, for generating a pixel code for each pixel position identified to be selectively tuned; and PA1 a look-up table, responsive to the pixel code received from said pixel code generator, for generating a pulse width position modulated output signal that is supplied to the print engine to control the output thereof.
In accordance with another aspect of the present invention, there is provided an apparatus for selectively tuning the output of an electronic printing system capable of producing printed output from a print engine in response to a bitmap image consisting of a plurality of input image signals, comprising:
One aspect of the invention is based on the observation of problems with conventional printing systems. Such systems, although they may be capable of rendering images received in a page description language (PDL) such as Interpress (Xerox Corp.) or Postscript (Adobe Corp.), typically render the image in accordance with the appearance determined as a result of the particular marking engine employed. However, such systems may result in user dissatisfaction because of an expectation of the appearance, based in part upon the WYSIWYG document preparation software and/or prior prints produced on other printers (e.g., dedicated workstation or desktop printers).
This aspect is based on the discovery of a technique that alleviates these problems by allowing the user to specify a desired appearance for the output print. This technique can be implemented, for example, by allowing the user to specify the level of density (lightness/darkness) with which the image is to be printed. A machine implementing the invention can include a laser-based electronic printing system, wherein the laser beam intensity or exposure level may be modified via pulse width position modulation (PWPM).